Difficulty levels

the number of givens: the more givens, the easier the sudoku - although the number of givens doesn't really tell about the difficulty

the position of the givens

As surprising as it may be, two sudoku grids can have the same cells filled with givens and at the same time be totally different, difficulty speaking.

Therefore the conclusion is that the complexity of a puzzle can only be measured in analyzing the methods required to solve it, and this analysis may differ from one person to another. That is what makes Sudoku so interesting!